[PATCH v2] docs/mm: add more warnings around page table access

From: Jann Horn
Date: Mon Nov 18 2024 - 11:50:34 EST


Make it clearer that holding the mmap lock in read mode is not enough
to traverse page tables, and that just having a stable VMA is not enough
to read PTEs.

Suggested-by: Matteo Rizzo <matteorizzo@xxxxxxxxxx>
Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
---
Changes in v2:
- improved based on feedback from Lorenzo
- Link to v1: https://lore.kernel.org/r/20241114-vma-docs-addition1-onv3-v1-1-ff177a0a2994@xxxxxxxxxx
---
Documentation/mm/process_addrs.rst | 46 +++++++++++++++++++++++++++++---------
1 file changed, 36 insertions(+), 10 deletions(-)

diff --git a/Documentation/mm/process_addrs.rst b/Documentation/mm/process_addrs.rst
index 1bf7ad010fc063d003bb857bb3b695a3eafa0b55..1d416658d7f59ec595bd51018f42eec606f7e272 100644
--- a/Documentation/mm/process_addrs.rst
+++ b/Documentation/mm/process_addrs.rst
@@ -339,6 +339,11 @@ When **installing** page table entries, the mmap or VMA lock must be held to
keep the VMA stable. We explore why this is in the page table locking details
section below.

+.. warning:: Page tables are normally only traversed in regions covered by VMAs.
+ If you want to traverse page tables in areas that might not be
+ covered by VMAs, heavier locking is required.
+ See :c:func:`!walk_page_range_novma` for details.
+
**Freeing** page tables is an entirely internal memory management operation and
has special requirements (see the page freeing section below for more details).

@@ -450,6 +455,9 @@ the time of writing of this document.
Locking Implementation Details
------------------------------

+.. warning:: Locking rules for PTE-level page tables are very different from
+ locking rules for page tables at other levels.
+
Page table locking details
--------------------------

@@ -470,8 +478,12 @@ additional locks dedicated to page tables:
These locks represent the minimum required to interact with each page table
level, but there are further requirements.

-Importantly, note that on a **traversal** of page tables, no such locks are
-taken. Whether care is taken on reading the page table entries depends on the
+Importantly, note that on a **traversal** of page tables, sometimes no such
+locks are taken. However, at the PTE level, at least concurrent page table
+deletion must be prevented (using RCU) and the page table must be mapped into
+high memory, see below.
+
+Whether care is taken on reading the page table entries depends on the
architecture, see the section on atomicity below.

Locking rules
@@ -489,12 +501,6 @@ We establish basic locking rules when interacting with page tables:
the warning below).
* As mentioned previously, zapping can be performed while simply keeping the VMA
stable, that is holding any one of the mmap, VMA or rmap locks.
-* Special care is required for PTEs, as on 32-bit architectures these must be
- mapped into high memory and additionally, careful consideration must be
- applied to racing with THP, migration or other concurrent kernel operations
- that might steal the entire PTE table from under us. All this is handled by
- :c:func:`!pte_offset_map_lock` (see the section on page table installation
- below for more details).

.. warning:: Populating previously empty entries is dangerous as, when unmapping
VMAs, :c:func:`!vms_clear_ptes` has a window of time between
@@ -509,8 +515,28 @@ We establish basic locking rules when interacting with page tables:
There are additional rules applicable when moving page tables, which we discuss
in the section on this topic below.

-.. note:: Interestingly, :c:func:`!pte_offset_map_lock` holds an RCU read lock
- while the PTE page table lock is held.
+PTE-level page tables are different from page tables at other levels, and there
+are extra requirements for accessing them:
+
+* On 32-bit architectures, they may be in high memory (meaning they need to be
+ mapped into kernel memory to be accessible).
+* When empty, they can be unlinked and RCU-freed while holding an mmap lock or
+ rmap lock for reading in combination with the PTE and PMD page table locks.
+ In particular, this happens in :c:func:`!retract_page_tables` when handling
+ :c:macro:`!MADV_COLLAPSE`.
+ So accessing PTE-level page tables requires at least holding an RCU read lock;
+ but that only suffices for readers that can tolerate racing with concurrent
+ page table updates such that an empty PTE is observed (in a page table that
+ has actually already been detached and marked for RCU freeing) while another
+ new page table has been installed in the same location and filled with
+ entries. Writers normally need to take the PTE lock and revalidate that the
+ PMD entry still refers to the same PTE-level page table.
+
+To access PTE-level page tables, a helper like :c:func:`!pte_offset_map_lock` or
+:c:func:`!pte_offset_map` can be used depending on stability requirements.
+These map the page table into kernel memory if required, take the RCU lock, and
+depending on variant, may also look up or acquire the PTE lock.
+See the comment on :c:func:`!__pte_offset_map_lock`.

Atomicity
^^^^^^^^^

---
base-commit: 1e96a63d3022403e06cdda0213c7849b05973cd5
change-id: 20241114-vma-docs-addition1-onv3-32df4e6dffcf

--
Jann Horn <jannh@xxxxxxxxxx>